In article ,
says...

I'm confused. The program to replace the RD-180 is focused on engines
with around 400,000 lb thrust at sea level. This focuses them on the
AR-1 (kerosene/LOC) and BE-4 (methane/LOX). Why are they not looking
at the RS-25 (LH2/LOX with similar thrust) or the Raptor engine
(methane/LOX)?

RS-25 is hella expensive and ULA already knows that LH2/LOX produces a
large, expensive, vehicle (e.g. Delta IV). So that's right out since
Delta IV is already flying (no development costs there). But do note
that ULA really wants to ax Delta IV in favor of Atlas V due to its high
cost.

Raptor (methane/LOX) isn't "fully baked" yet (BE-4 is ahead of it).

Seems like we're getting a lot of different engines when it might be
more efficient to settle on just a couple.

AR-1 is a "backup" engine at this point since it's so far behind BE-4 in
both schedule and (estimated) per unit price. But, AR-1 is about the
right size for two of them to be a "drop-in" replacement for RD-180 on
Atlas V. So, if ULA stumbles on Vulcan, an AR-1 engined Atlas V might
be a good stop-gap measure.

At any rate, Aerojet Rocketdyne is being paid good money to develop AR-
1. Even if it meets the same fate as J-2X, they're getting money now
which helps keep the company alive.

Jeff
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